1. Field of the Invention
The present invention relates to an electric charge type sensor, such as a piezoelectric type acceleration sensor and a pyroelectric type infrared sensor.
2. Description of the Related Art
Piezoelectric type acceleration sensors, pyroelectric type infrared sensors or the like are known as sensors which output a detection signal in the form of an electric charge. In these sensors, if a detection element which consists of a piezoelectric element, a pyroelectric element, or the like detects an acceleration, an infrared ray, etc., an electric charge will be generated on the surface of the detection element. Since this electric charge amount is so small (for example, 0.01 to several hundreds pico-coulombs), it is necessary to amplify this charge and output it as a voltage signal etc. For example, Japanese Laid-Open Patent Publication No. 8-338781 discloses that the output of a detection element is amplified with a charge amplifier.
FIG. 1 shows a circuit diagram used in a conventional acceleration sensor having a charge amplifier. In this acceleration sensor, a feedback capacitor 14 and a feedback resistor 13 are connected in parallel between an output terminal and an inversion input terminal of an operational amplifier 11. A non-inversion input terminal of the operational amplifier 11 is applied with a reference voltage V.sub.ref. An output terminal of a piezoelectric element 2 which is a detection element for detecting an acceleration is connected to the inversion input terminal of the operational amplifier 11, and the other terminal of the piezoelectric element 12 is applied with the reference voltage V.sub.ref.
In the acceleration sensor having such a construction, the piezoelectric element 12 is structured such that, when an acceleration or a vibration is applied to the piezoelectric element 12, a charge in accordance with the degree of the acceleration or vibration is generated at the output terminal. The operational amplifier 11 outputs a voltage V.sub.0 in accordance with an amount of the charge Q generated at the output terminal of the piezoelectric element 12. The voltage V.sub.0 is expressed by the following equation: ##EQU1## where C.sub.f is a capacitance of the feedback capacitor 14, and R.sub.f is a resistance of the feedback resistor 13. V.sub.0(s) represents a Laplace transform function denoted as a algebraic equation with respect to s.
FIG. 2 shows a frequency characteristics of the acceleration sensor defined by the equation (1). The frequency characteristic such as that shown in FIG. 2 is characterized by the output voltage in the flat region and a cut-off frequency f.sub.c. The cut-off frequency f.sub.c is defined by the frequency f where the output voltage is reduced by 3 dB from the output voltage .vertline.V.sub.0 .vertline.(.vertline.Q/C.sub.f .vertline.) in the flat region. In the case where the acceleration sensor has the circuit as shown in FIG. 1, the cut-off frequency f.sub.c is expressed by the following equation (2). ##EQU2##
In the acceleration sensor, it is preferable that the cut-off frequency is made small in order to broaden the sensitive region of the acceleration or vibration by enlarging the flat region of the frequency characteristics.
As shown in the equation (2), it is necessary to increase the capacitance C.sub.f of the feedback capacitor 14 or the resistance R.sub.f of the feedback resistor 13 in order to lower the cut-off frequency. However, if the capacitance C.sub.f is increased, there arises a problem that the acceleration sensor becomes unstable in operation and is likely to cause an oscillation. Further, the output voltage V.sub.0 of the flat region becomes so small that the sensitivity degrades. On the other hand, in the case where the resistance R.sub.f is increased, the resistance R.sub.f must be greater than 100 M.OMEGA.. Since the resistor having such a high resistance is expensive, there arises a problem that the acceleration sensor becomes expensive.
For the forgoing reasons, there is a need for an electric charge type sensor which has a small cut-off frequency without using a feedback resistor having a high resistance, which outputs a large output voltage and which operates stably.